Various techniques by which a p-type region is formed on a surface of a silicon semiconductor substrate have been conventionally known, including dopant host, counter BN and thermal decomposition techniques.
The dopant host technique is a technique which involves positioning a wafer of B2O3-containing glass-ceramic and a semiconductor wafer parallel to each other in spaced confronting relationship, allowing B2O3 vaporized from the glass-ceramic to deposit on the semiconductor wafer and then thermally diffuse therein (see, for example, Patent Document 1). The counter BN technique is almost the same as the dopant host technique but differs therefrom by the use of a boron nitride wafer which has been subjected to an activation treatment (converting BN to B2O3), instead of using the glass-ceramic. The thermal decomposition technique is a technique which involves vaporizing liquid-form BCl3, BBr3 and others through bubbling and allowing the vapor to deposit on a preheated semiconductor wafer and then decompose to obtain a deposition film of B2O3, followed by thermal diffusion.
According to the procedure disclosed in Patent Document 1, the dopant host technique can be carried out at a lower process cost compared to the case of using boron nitride, because there is no need to perform the activation treatment when a dopant host is used. The thermal decomposition technique involves deposition of a gas on a semiconductor wafer and accordingly raises a problem that the deposit variation becomes large when B2O3 is diffused into a large-sized wafer. However, diffusion of B2O3 is maintained at a low degree of variation by the dopant host technique in which a silicon wafer and a glass-ceramic wafer having the same areal size are positioned in a confronting relationship and then subjected to a heat treatment.
Boron dopants for a semiconductor have been conventionally proposed for doping a silicon substrate or the like with boron, including those produced by sintering a boron nitride powder and those of crystallized glass type that are produced by crystallizing a molded glass containing boron and then cutting it into the wafer form (see, for example, Patent Document 2). A doping process is employed which involves heating a surface of a boron dopant for semiconductor in an oxidizing atmosphere to vaporize B2O3 and allowing B2O3 to deposit on a surface of a substrate located opposite to the boron dopant's surface, such as a silicon wafer, and then diffuse into the substrate.
The boron dopant for a semiconductor is required to have the following properties; (1) it can liberate a boron vapor from its surface when heated so that boron is allowed to diffuse sufficiently into a substrate, such as a silicon wafer, located opposite to the dopant, (2) it is durable for repeated use, (3) it can liberate a consistent amount of the boron vapor at each use, and (4) it can be readily processed into the same shape as the substrate.    Patent Document 1: Japanese Paten Laid-Open No. Sho 52-55861    Patent Document 2: Japanese Paten Laid-Open No. 2002-93734